Paste solids are measured in connection with monomer and polymer manufacture in order to control the processes and characterize product which is sold in intermediate or finished form. Known techniques are gravimetric in nature and are difficult to reproduce especially if a relatively volatile solvent such as methanol is used as is the case in connection with processing vinyl acetate monomer into polyvinyl alcohol. Conventional techniques involve baking a paste sample in an oven at 150° C. or so to drive off the solvent.
Numerous types of optical sensors sensitive to changes in refractive index have been described in the art. These include devices which operate by measuring optical energy internally reflected at an interface with a surrounding medium. Optical fibers may serve to direct light onto the interface and may also serve as the optical detectors themselves. These devices are relatively inexpensive to produce, immune from electromagnetic interference and intrinsically safe in explosive environments.
A fiber optic refractometer does not require light to pass through the process liquid, but offers a means of continuously measuring RI values. The efficiency with which optical fibers transmit light is determined by the disparity of RI that exists between the core and cladding materials. It follows that such a device could be used as a refractometer if the process liquid of interest became the “cladding” about a glass core. By measuring the efficiency at which such a fiber transmitted light energy, the RI of the liquid cladding could be determined. The concept of attenuated total reflectance (ATR) forms this category of fiber optic refractometers. (See Kapany, N. S., and J. N. Pike, “Fiber Optics, part IV, a photorefractometer,” Journal of the Optical Society of America, vol. 47, no. 12, 1957, pp. 1109-1117.) In these instruments, a conical beam of light with a uniform intensity, I watts/steradian, excites a glass rod or fiber. The transmitted light is then measured by a photosensitive device.
A variation of the ATR fiber optic refractometer uses a laser beam incident on the end of the glass rod. (See David, et al., “Design, development and performance of a fiber optics refractometer: Application to HPLC,” Review of Scientific Instruments, vol. 47, no. 9, 1976, pp. 989-997; also, U.S. Pat. No. 3,999,857, J. D. David, D. A. Shaw & H. C. Tucker, “Refractive Index Detector,” 1976.) The beam angle into the rod is adjusted via a mirror moved by a micrometer until the edge of the “cone of acceptance” (i.e., the numerical aperture or NA) is found. Multiple reflections of the light propagating down the fiber make the transition very sharp. The micrometer reading correlates to the NA, and n.sub.1 can be calculated from equation (4a). The instrument locates the sharp light transition at the edge of the NA, but its output drops to a low, constant level once the incident beam angle exceeds the NA.
Fiber optic refractometers based on Fresnel's equations have also been designed. (See Meyer, M. S., and G. L. Eesley, “Optical Fiber Refractometer,” Review of Scientific Instruments, vol. 58, no. 11, 1987, pp. 2047-2048.) Monochromatic light is transmitted down a single mode fiber and reflects off the far end of the fiber, immersed in the process liquid. The core at that end of the fiber is polished smooth, perpendicular to the fiber axis. Fresnel reflections from the core/liquid dielectric interface are transmitted back through the fiber to a photo sensor.
Fiber optic refractometers using bent fibers have also been developed. (See Golunski, W., et al., “Optical fiber refractometer for liquid refractive index measurement,” Proceedings of the SPIE—Optical Fibers and Their Applications V, vol. 1085, 1990, pp. 473-475.)
U.S. Pat. No. 5,311,274 to Charles F. Cole Jr., May 10, 1994, describes an ATR type fiber optic refractometer suitable for use in determining on-line measurements of the hydrogenation state of edible oils during a partial hydrogenation process. This refractometer does not require light to pass through the process fluid and is therefore unaffected by the presence of light diffusing particulate matter in the process fluid.
U.S. Pat. No. 5,396,325 assigned to the Mercury Iron and Steel Company, Mar. 7, 1995, describes a refractometer of the fiber-optic Fresnel “reflectance” type suitable for measuring refractive index provided with a sensor element and first and second optical fibers coupled to the sensor element. Optical energy incident at an angle to a surface less than the critical angle is governed by the Fresnel reflectance equation:
  R  =            1      /      2        ⁢          (                                                  sin              2                        ⁡                          (                                                θ                  i                                -                                  θ                  r                                            )                                                          sin              2                        ⁡                          (                                                θ                  i                                +                                  θ                  r                                            )                                      +                                            tan              2                        ⁡                          (                                                θ                  i                                -                                  θ                  r                                            )                                                          tan              2                        ⁡                          (                                                θ                  i                                +                                  θ                  r                                            )                                          )      where θi is the angle of incidence of the optical energy and θr is the angle of the refracted optical energy. At a specific angle of incidence, if the refractive index of the covering medium approaches the refractive index of the glass layer, the percent of reflectance decreases and more optical energy passes into the covering medium. Since the change in the reflected optical energy is dependent on changes in the angle of incidence and the refractive index of the covering medium, the above equation may be used as the basis of a detection scheme.
While fiber optic refractometers have been used for measuring concentration, such as protein concentration in dilute agitated aqueous solutions; it is conventionally believed that a fiber optic probe would not function well when submersed in viscous paste, due to air bubbles and an inability to circulate fluid about the probe.
It was unexpectedly found in accordance with the present invention that concentrated solutions or pastes are amenable to concentration measurement by way of a calibrated refractometer. The method provides essentially real time concentration measurement as opposed to gravimetric techniques which can take an hour or more. The method is also more accurate since a major source of error, unaccounted for evaporation, is minimized.